This invention relates to apparatus for scavenging charged particles from a developed electrostatic image.
In electrography, it is common to develop a latent electrostatic image borne by a dielectric or photoconductive recording element by contacting such image with a developer mix comprising magnetically attractive carrier particles and pigmented, thermoplastic particles, commonly known as toner. When mixed together and agitated, the toner and carrier particles become triboelectrically charged to opposite polarities, the polarity of the toner usually being opposite that of the electrostatic image being developed. When the developer mix is applied to the latent image (e.g. by a conventional magnetic brush applicator), the electrostatic forces associated with the image overcome the triboelectric attraction between toner and carrier, the result being that the toner is stripped from the carrier and applied to the image to effect development thereof. Thereafter, the partially denuded carrier particles are returned to a developer sump to be replenished with toner.
In conventional electrographic developing apparatus, it is common to electrically bias a development electrode, positioned directly opposite and in close proximity to the charge image, to a voltage intermediate that of the image and background areas constituting the charge image. For example, assuming those image areas which are to be developed with toner are charged to a level of, say, -500 volts, and the remaining areas (i.e. the backgound areas) are charged to a level of, say, -50 volts, the development electrode might be biased to -200 volts. In this manner, the positively charged toner particles will be attracted more toward the development electrode than toward the background areas of the electrostatic image. While this biasing scheme has the advantageous effect of preventing the deposition of toner particles in the background areas of the image, it has the undesirable effect of causing carrier particles to deposit on such areas and be carried out of the development zone with the developed image. This so-called "carrier pick-up" has an adverse effect on image quality and, worse yet, can produce deleterious effects in the form of scratches or scars in the surface of the recording element and other copier components, such as transfer and fusing rollers.
Heretofore, many schemes have been devised for scavenging charged carrier particles from the developed surface of an electrographic recording element. For example, in U.S. Pat. No. 4,349,270, there is disclosed a carrier scavenging apparatus comprising an electrically conductive member which is positioned downstream of the development zone in close proximity with the recording element's surface. Means are provided for DC biasing the conductive member to a voltage which renders it more attractive to the carrier particles than to the recording element. In this manner, the carrier particles are pulled from the recording element by stronger electrostatic forces. To remove the scavenged particles from the biased scavenging member, the latter is located relative to a magnetic brush applicator of the rotating core variety such that the particles are influenced by the rapidly changing magnetic field produced by the rotating magnetic core. As the core rotates, the scavenged carrier particles are advanced by magnetic forces along the surface of the scavenging member to a position at which they return to the development sump under the force of gravity.
In scavenging apparatus of the above type, it can be difficult to rid the biased scavenger member from the electrostatically-attracted carrier particles. This is especially true when a DC bias is applied to the scavenging member. Moreover, as the scavenging member becomes contaminated with the scavenged particles, the applied bias becomes increasingly less effective in attracting the particles to the scavenging member.